Neptune

Neptune (also known as Docay or Nebtuun) is the eighth planet from the Sun, measured from the centre of the solar system. It is the most distant known planet since Pluto was removed from the list of major planets in 2006. Neptune is one of the giant planets composed of gas and ice, with no solid surface on which to stand.

Neptune has a blue colour that comes from the methane gas in its atmosphere, which absorbs red light and reflects blue light. Its core is roughly four times the mass of Earth, and by volume it is significantly heavier than Earth. Neptune takes about 165 Earth years to orbit the Sun.

Discovery

Neptune is the only planet that was first predicted by calculation before being observed through a telescope. Mathematicians, including Urbain Le Verrier and John Couch Adams, studied the irregularities in the orbit of Uranus. They concluded that another unseen planet was affecting it. In 1846, astronomers at the Berlin Observatory observed the planet near the predicted location.

Structure and atmosphere

Neptune is largely composed of hydrogen, helium, and methane, with what is believed to be a mantle beneath consisting of water, ammonia, and methane ice—hence the term 'ice giant'. The interior is thought to contain a rocky and iron core. Neptune's atmosphere contains extremely strong winds, among the fastest in the solar system. During the 1980s, a large dark spot was observed (the Great Dark Spot), which later disappeared.

Moons and rings

Neptune has many moons, the largest being Triton. Triton orbits in a direction opposite to the planet's rotation, which suggests it may originally have been an object from the outer reaches of the solar system before being captured by Neptune's gravity. Neptune also has faint rings close to it, unlike the prominent rings of Saturn.

Exploration

So far only one spacecraft has visited Neptune: Voyager 2, which passed by in 1989. The images and data it transmitted form the basis of most current knowledge about the planet's atmosphere, moons, and rings. Since then, observations have relied on Earth-based telescopes and space telescopes such as Hubble.